Reference is made to copending patent application Ser. No. 07/619,579, filed Nov. 29, 1990, entitled "Circuit for Generating or Demodulating a Square Wave and Other Wave Forms", filed by the applicant of the present application and assigned to the assignee of the present application. The disclosure of said copending application Ser. No. 07/619,579 is hereby incorporated herein by reference.
It is well known to use transducers such as strain gauges, thermistors, or linear variable differential transformers (LVDT's) to measure such physical quantities as strain, displacement, position, temperature or force. Often an excitation signal is applied to the transducers and the quantity to be measured modulates the signal. The modulated signal is then detected, analyzed and/or quantified, etc. For example, it is known to use a strain gauge comprising a resistor bridge circuit for measuring a change in a physical dimension of a body. One common application of such a strain gauge is in the load cell of a weighing scale. In many cases, a constant d.c. voltage is applied to the bridge circuit and the output voltage is then processed to provide an indication of the physical strain to be measured. However, use of d.c. excitation is subject to a number of errors, arising from such causes as thermocouple effects, 1/f noise, d.c. drift in the electronic components and line noise pick-up.
It is also known to avoid these difficulties by exciting the bridge with a sinusoidal a.c. waveform. The bridge output is then amplified and synchronously demodulated, and the error-causing noise can then be removed by a low pass filter. However, a disadvantage of this approach is the relatively high cost for components required to generate and demodulate a sinusoid waveform with great precision.